bug #6845 Fixed the extension of a logging article (javiereguiluz)

This PR was merged into the 2.7 branch.

Discussion
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Fixed the extension of a logging article

I was trying to figure out why this article didn't appear in the "Learn More" section of the main article at http://symfony.com/doc/current/logging.html  The reason? The article used `.md` extension instead of `.rst`

Commits
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ad3339c Fixed the extension of a logging article

Author: wouterj